1. Field of the invention
The present invention relates to a still picture display apparatus and an external storage device used in the apparatus. More specifically, the present invention relates to a still picture apparatus which can scroll still pictures in a picture processing device, for example, a personal computer, a video game machine or the like.
2. Description of the Prior Art
For example, Japanese patent publication No. 2-7478 published on Feb. 19, 1990 (corresponding to U.S. Pat. No. 4,824,106) discloses a picture display apparatus which can scroll a still picture. In the prior art, scrolling on an entire monitor screen or partial scrolling of each line is possible.
For example, when a part of still picture divided in the vertical direction is scrolled in the arrow direction as shown in FIG. 18, by using a game program, a vertical direction position (V position) on the monitor screen to be scrolled may be detected, and a horizontal offset value of lines on and after the V position may be changed to a horizontal blanking period (85.times.18 nsec).
On the other hand, in order to obtain variety of scroll of still pictures, it is desirable to divide the monitor screen in the horizontal direction and scroll a part thereof in the vertical direction as shown in FIG. 19.
In the prior art, in order to achieve scrolling shown in FIG. 19, a horizontal direction position (H position) on the monitor screen may be detected with a program in progress, and an offset value in the vertical direction may be changed on reaching the H position. However, in order to adopt this method, it is necessary to rewrite the offset value in the V direction during a scanning period. For example, in order to move one character (8.times.8 dots) in the V direction, the value must be written during a display period of one character, e.g., 8.times.18 nsec. It is practically impossible to rewrite the offset value in the V direction in such a short time. Therefore, in the prior art, a partial scrolling as shown in FIG. 19 has not been realized.
Also, scrolling is disclosed, for example, in Japanese patent publication No. 63-32472 published on Jun. 30, 1988. In this other prior art, an offset value from an MPU is written into a scroll index latch, and the offset value and a count value of a horizontal synchronous counter are calculated by an adder and an address is designated to read and delay the data thereby to realize a scrolling. Also in this prior art, an initial value and the offset value must be rewritten for scrolling by a CPU or MPU. Thus, when the CPU or MPU is concerned with scrolling, in order to rewrite data within a short time as mentioned above, the CPU or MPU of high speed is required. Under such conditions, this method cannot be used for a video game machine or the like which is accompanied by a condition that it must be cheap.
Furthermore, there is Japanese patent application laid-open No. 58-81065 laid open on May 1, 1983. This prior art carries out similar processing to that of Japanese patent publication No. 63-32472 concerning a plurality of still picture cells, and therefore the same problems remain.